Update TimeStamp to be time.Duration not integer

[tylerwince]

The current structs have TimeStamp set to be an int when it is probably better served to be a time.Duration object.

We will likely need to update the UnmarshalJSON of some of these to get everything to work correctly. Something like the following:

type HarvestInfo struct {
	TimeStamp  timedur
	Difficulty int64
	TotalFee   int64
	ID         int64
	Height     int64
}

type timedur struct {
	time.Duration
}

func (td *timedur) UnmarshalJSON(b []bytes) error {
    timestamp, err := strconv.ParseFloat(string(b), 0)
    if err != nil {
        return err
    }
    td.Duration = time.Duration(timestamp) * time.Second
    return nil
}

Not sure this is the best approach, but it is tested in playground and works
https://play.golang.org/p/Voi-DFkZ-qh